Porous cordierite ceramics and methods of making such ceramics are generally known. For example, U.S. Pat. No. 3,950,175 issued Apr. 13, 1976 to Lachman et al. entitled "Pore Size Control in Cordierite Ceramics" discloses a porous cordierite ceramic having at least 20 percent of the pres greater than 10 microns in diameter and a mean pore size in the range of 3.3 to 12.6 microns in diameter. The control of pore-size is accomplished by substituting pyrophillite, kyanite, quartz or fused silica in the raw material batch as replacements for all or a portion of the talc or clay.
In U.S. Pat. No. 4,416,675 issued Nov. 22, 1983 for "High Capacity Solid Particulate Filter Apparatus," Montierth describes a honeycomb filter system of thin interconnected porous walls made of primarily cordierite ceramics. The thin, porous walls having a predetermined porosity and pore size maximizes the ability of the filter to entrap a significant portion of solid particulates while assuring adequate fluid flow through the filter. The predetermined porosity, i.e., the percentage of total volume not occupied by a solid material, required to accomplish the filtering purpose of the Montierth patent is disclosed as being in the range of 30 to 70 percent by volume, with a mean pore size greater than about 1 micron. For diesel exhaust gas particulate filtration, the mean pore size ranges from about 20 to 60 microns in diameter.
The cordierite ceramic disclosed in U.S. Pat. No. 4,280,845 issued July 28, 1981 to Matsuhisa et al. for a "Cordierite Ceramic" is specifically adapted for a catalyst support system having catalyst adhering ability. The mean pore size of the cordierite ceramic is disclosed as being in the range of 2 to 50 microns. Control of pore size is accomplished by the use of a magnesia raw material having an average particle size of 5 to 150 microns in the starting composition.
Typically, insulation from a heat source is desirable to prevent injury or discomfort to a user if the user is likely to come into physical contact with the heat source. Moreover, an insulating retainer for a hand-held heat source or one that is movable is preferably lightweight to ensure easy manipulation, yet sufficiently strong to maintain its integrity during normal use.
The problem of maintaining or increasing strength and insulation while minimizing weight has not been addressed by previous procedures relating to cordierite ceramics. Generally, strength and thermal conductivity can be increased by increasing the density of a material. Densification, however, such as by reduction of porosity, leads to a corresponding increase in weight of the finished part.
Previous approaches of providing insulation and containment of hand-held heat source devices have used a variety of materials. For example, U.S. Pat. No. 2,907,686 issued Oct. 6, 1959 to Siegel discloses a sheathing overwrap comprised of concentrated sugar solutions, hard gum or resins to contain a heat source described therein.
U.S. Pat. No. 3,943,941 issued Mar. 16, 1976 to Boyd et al. discloses the use of a non-porous carbon mat as a non-combustible retainer to hold a synthetic carbon fuel used as a heat source to activate a product.
Finally, U.S. Pat. No. 3,200,819 issued Aug. 17, 1965 to Gilbert discloses a heating element encased in an external tube made of plastic or fiberglass. This patent further discloses an interior lining comprised of ceramic used to insulate an external tube from a heating element. Although the Gilbert patent generally discloses the use of ceramic material for insulation purposes, it does not describe nor teach the specific properties or qualities desirable, e.g., for effective insulation.
Accordingly, it would be advantageous to find a lightweight, strong material capable of providing effective insulation from a heat source to prevent injury or discomfort to a user.